CHARACTERISTICS OF SURFACE-WAVES GENERATED BY EVENTS ON AND NEAR THE CHINESE NUCLEAR TEST SITE

Our goal is to characterize surface waves at intermediate periods (5-3 0 s) generated by nuclear explosions and natural earthquakes occurring in and around the Chinese nuclear test site at Lop Nor observed at ep icentral distances between 10 degrees and 20 degrees. We present obser vations of Rayleigh- and Love-wave group velocities, spectral amplitud es and polarizations for eight nuclear explosions and nine earthquakes recorded at IRIS and GEOSCOPE broad-band digital stations and at the Kirghiz Telemetered Seismic Network (KNET). The wave paths studied cro ss complex geological regimes. Waveforms are extremely complicated, ye t observations made with frequency-time analysis are robust and repeat able. (1) Group velocity and spectral amplitude measurements vary stro ngly across the studied region, but display understandable systematics related to known tectonic features. (2) Both group velocity and spect ral amplitude measurements across the KNET array are generally similar at periods above 10s, but differences are striking at shorter periods . Stacking methods dependent on waveform coherence across the network should only succeed at periods above 10s for this large-scale array. ( 3) Group velocity measurements for earthquakes and nuclear explosions that share a common wave path agree well, but spectral amplitudes diff er appreciably, especially for Rayleigh waves. Typically, earthquake s pectra are enriched in longer periods (12-20 s) relative to explosion spectra for events of the same magnitude. (4) Higher modes are observe d on frequency-time diagrams for Rayleigh waves generated by earthquak es, but are not observed for explosions. (5) Multipathing is observed in both frequency-time diagrams and polarization analyses, but, again, is qualitatively understandable in terms of known structural features . In particular, the channelling of Love waves by the Tarim Basin is d ocumented. (6) Large-amplitude Love waves are generated by nuclear exp losions at Lop Nor, indicative of significant tectonic release. We dis cuss the relevance of the differences in spectral amplitudes between e arthquakes and explosions and of the appearance of higher modes on ear thquake frequency-time diagrams to the problem of discriminating nucle ar explosions from earthquakes.